Add obsolete message for renaming Layout/FirstParameterIndentation

Follow up #6982.

This PR adds obsolete message for renaming `Layout/FirstParameterIndentation`.

The following is when `Layout/FirstParameterIndentation` is specified in
.rubocop.yml.

## Before

```console
% rubocop
Warning: unrecognized cop Layout/FirstParameterIndentation found in
.rubocop.yml
```

## After

```console
% rubocop
Error: The `Layout/FirstParameterIndentation` cop has been renamed to
`Layout/IndentFirstArgument`.
(obsolete configuration found in .rubocop.yml, please update it)
```

lib/rubocop/config.rb

@@ -52,6 +52,9 @@ class Config
         'The `Lint/SpaceBeforeFirstArg` cop has been removed, since it was a ' \
         'duplicate of `Layout/SpaceBeforeFirstArg`. Please use ' \
         '`Layout/SpaceBeforeFirstArg` instead.',
+      'Layout/FirstParameterIndentation' =>
+        'The `Layout/FirstParameterIndentation` cop has been renamed to ' \
+        '`Layout/IndentFirstArgument`.',
       'Layout/SpaceAfterControlKeyword' =>
         'The `Layout/SpaceAfterControlKeyword` cop has been removed. Please ' \
         'use `Layout/SpaceAroundKeyword` instead.',